Time Zones

Time zones give specific areas on the Earth a time of day that is earlier or later than the neighboring time zones. This is
because when it is day-time on one side of the Earth, it is night-time on the other side. There are 39 time zones dividing
the Earth into different times, each with its own name.

Greenwich Mean Time (GMT) is now called UTC (Coordinated Universal Time). All other parts of the world are offset
(plus or minus) according to their longitude. Most of the zones are offset by a full hour, but there are some offset by half
an hour or 45 minutes.

Time in the United States

Time in the United States is divided into nine standard time zones, with most of the United States observing daylight
saving time for approximately the spring, summer, and fall months.

Daylight saving time (DST) or summer time is the practice of advancing clocks during summer months by one hour so that
in the evening daylight is experienced an hour longer, while sacrificing normal sunrise times. Typically, regions with
summer time adjust clocks forward one hour close to the start of spring and adjust them backward in the autumn to
standard time.

The time zone boundaries and DST observance are regulated by the Department of Transportation. Official and highly
precise timekeeping services (clocks) are provided by two federal agencies: the National Institute of Standards and
Technology (NIST) (an agency of the Department of Commerce); and its military counterpart, the United States Naval
Observatory (USNO). The clocks run by these services are kept synchronized with each other as well as with those of
other international timekeeping organizations.

United States time zones:

Atlantic Time Zone
Eastern Time Zone
Central Time Zone
Mountain Time Zone
Pacific Time Zone
Alaska Time Zone
Hawaii–Aleutian Time Zone
Samoa Time Zone (UTC-11)
Chamorro Time Zone (UTC+10)

From east to west, the times zones of the contiguous United States are:

Eastern Time Zone: (Zone R), which comprises roughly the states on the Atlantic coast and the eastern two thirds of
the Ohio Valley. Central Time Zone: (Zone S), which comprises roughly the Gulf Coast, Mississippi Valley, and Great Plains. Mountain Time Zone: (Zone T), which comprises roughly the states that include the Rocky Mountains. Pacific Time Zone: (Zone U), which comprises roughly the states on the Pacific coast, plus Nevada and the Idaho
panhandle.

Zones used in states beyond the contiguous U.S.

Alaska standard time zone: (AKST; UTC-09; Zone V), which comprises most of the state of Alaska. Hawaii-Aleutian standard time zone: (or unofficially Hawaii Standard Time: HST) (HAST; UTC-10; zone W), which
includes Hawaii and most of the length of the Aleutian Islands chain (west of 169°30'W).